1. Field of the Invention;
The present invention relates to a variable capacitance device. More specifically, the present invention relates to a variable capacitance device particularly suited for use in a digital channel selecting apparatus, wherein a plurality of capacitance elements are coupled in parallel, each constituting a portion of a tuning circuit, such that each capacitance element is individually coupled to a corresponding switching device which is operable responsive to a digital control signal, whereby the total capacitance value associated with the tuning circuit is varied to achieve channel selection.
2. Description of the Prior Art;
As well known, a channel selecting apparatus or a tuning apparatus comprises at least one tuning circuit including inductance and capacitance components. For the purpose of channel selection in such a channel selecting apparatus, either or both of the inductance and capacitance must be varied to attain a different tuning frequency by the tuning circuit. Most typically, the capacitance component is structured to be variable while the inductance component is fixed. Such a capacitance component structured to be variable, or a variable capacitance device, is required in various applications, such as an LC or RC oscillator, for example, wherein a desired frequency is attained through selective variation of the capacitance value of such a variable capacitance device.
A typical scheme so far adopted for varying the capacitance value of such a variable capacitance device is to vary a reverse bias voltage to be applied to a voltage controlled variable capacitance diode coupled in the device, thereby to vary the capacitance across the diode.
Recently, a variable capacitance device comprising a plurality of series connections coupled in parallel with each other each including a capacitance element and a switching diode was developed. In using such a variable capacitance device, the same is connected such that the total capacitance is varied by selectively applying a digital control signal to each of the switching diodes, thereby to selectively render effective a desired capacitance element or elements.
The above described voltage controlled variable capacitance diode and the above described variable capacitance device comprising a plurality of series connections coupled in parallel with each other each including a capacitance element and a switching diode are particularly suited for the tuning apparatus or a channel selecting apparatus. FIG. 1 shows a block diagram of one example of a typical prior art tuner. In particular, FIG. 1 is a block diagram showing an outline of a tuner or a channel selecting apparatus for use in a typical television receiver. As is well known, the tuner 1 comprises an input tuning circuit 2, an inter-stage tuning circuit 3, a local oscillator 4 and a mixer 5 for mixing the high frequency signal from the inter-stage tuning circuit 3 with the local oscillation signal from the local oscillator 4 to provide an intermediate frequency signal through superheterodyne detection.
In a channel selecting operation by means of the tuner 1, the input tuning circuit 2, the inter-stage tuning circuit 3 and the tuning circuit included in the local oscillator 4 must be adjusted to a desired tuning frequency determinable depending on the channel to be selected.
FIG. 2 shows a schematic diagram of such a tuning circuit that could be implemented using the above described variable capacitance device comprising a plurality of series connections coupled in parallel with each other each including a capacitance element and a switching diode. It is needless to say that such a tuning circuit as shown in FIG. 2 may be employed as the input tuning circuit 2, the inter-stage tuning circuit 3, and the tuning circuit included in the local oscillator 4 shown in FIG. 1, for example.
FIG. 2 shows an example of a tuning circuit that constitutes the background of the invention and can be theoretically utilized in a very high frequency or VHF application. Thus, the VHF tuning circuit shown comprises series connected coupling capacitors CC1 and CC2, an inductance coil L coupled in parallel between these coupling capacitors CC1 and CC2 and the above described variable capacitance device C coupled in parallel between these capacitors CC1 and CC2. The variable capacitance device C comprises a plurality of capacitance elements C1, C2, C3, . . . Cn, and a corresponding plurality of switching diodes D1, D2, D3, . . . Dn, each connected in series with the corresponding one of the above described capacitance elements C1, C2, C3, . . . Cn, respectively, thereby to constitute a corresponding plurality of series connections in parallel with each other, each including a capacitance element and a switching diode. The junctions of the respective series connections are coupled through corresponding resistors R1, R2, R3, . . . Rn to output terminals t1, t2, t3, . . . tn respectively, of a digital signal generator 7. The switching diodes D1, D2, D3, . . . Dn are each responsive to a corresponding individual digital control signal from the digital signal generator 7 to be rendered conductive, whereby the capacitance element coupled to the switching diode now rendered conductive is rendered effective or selected as a capacitance to constitute a portion of the tuning circuit as desired. For the purpose of facility of understanding, assuming that only the switching diodes D1 and D2 are selected as a function of the control signals from the digital signal generator 7, the total capacitance of the variable capacitance device C can be simply calculated as (C1+C2). Thus, the total capacitance of the variable capacitance device C can be varied in a digital manner or a stepwise manner as a function of the digital control signal.
Meanwhile, in order to enable selection of many channels using the above described tuning circuit, a correspondingly increased number of capacitance elements are required in such a variable capacitance device. In view of an increased number of capacitance elements in a variable capacitance device, it is desired that such a variable capacitance device is implemented in an integrated circuit. Nevertheless, it is rather difficult to attain a uniform characteristic throughout the wide range of capacitance values by such a variable capacitance device as implemented in an integrated circuit. Some other problems are encountered in such an integrated circuit variable capacitance device. For these reasons, it is not so easy to implement such a variable capacitance device implemented in an integrated circuit.